1. Field of the Invention
The present invention relates to a cationic electrodeposition using a specific cationic resin.
2. Description of the Prior Arts
An electrodeposition process by using a water soluble resin or a water dispersible resin has been developed for a base coating (the first prime coat) on a car. Novel resins have been further developed for the purpose of improvement of anticorrosive property. However, it has not attained to provide a satisfactory resin.
In the conventional anionic electrodeposition process, a metal substrate is connected to an anode whereby the metal substrate and the pretreated layer thereof are dissolved. The anticorrosive property thereof has not been satisfactory.
It has been proposed to provide a cationic electrodeposition-coating resin and process which do not cause dissolution of a metal substrate. However, resins produced hitherto are hardly, practially applicable in view of an anticorrosive property, a mechanical property and a stability for storing as a coating composition.
The inventors have studied on electrodeposition processes which have not the above-mentioned disadvantages. It has been found to overcome these disadvantages by a cationic electrodeposition by using a paint comprising a cationic aqueous urethane resin. The inventors have disclosed the process for producing the cationic aqueous urethane resins and the electrodepositions in Japanese Examined Patent Publication No. 17234/1975 and 2491/1976.
The inventors have further studied. As a result, it has been found that a coating composition comprising a cationic aqueous urethane resin having quaternary ammonium groups imparts a suitable pH and excellent rupture voltage, coulomb efficiency and dispersion stability.
The cationic electrodeposition using an electrodeposition coating composition comprising a resin having quaternary ammonium groups is disclosed in Japanese Unexamined Patent Publication No. 65329/1978. The resin used in this method is produced by quaternizing a polymer having tertiary amino groups with 1,2-epoxy compound. In accordance with this process, the quaternizing reaction is not easily attained as a common problem for a chemical reaction of a polymer and a high temperature is required and a reproduction is not satisfactory.
When a blocked urethane resin is used as a starting material, especially an urethane resin blocked by a blocking agent which is disassociated at low temperature is used, the quaternizing reaction causes a crosslinking reaction of the resin by the disassociation of the blocked group. Therefore, it is impossible to employ the process for producing the resin described in said patent application. The reactivity of the quaternization is remarkably low and accordingly, most of 1,2-epoxy compound remains in the reaction mixture to cause an adverse effect for a long stability. The residual 1,2-epoxy compound can be removed, however, it is disadvantageous to separate 1,2-epoxy compound from a viscous polymer in view of an industrial operation.
The inventors have further studied and have found that a coating composition having highly stable and high quality can be obtained by using a cationic aqueous urethane resin produced by using a hydroxyl compound having quaternary ammonium group. The present invention has been attained by the finding.